Question regarding classified ads: how is home-page placement rate determined? It seems like only the most recent 5-10 ads get rotated in when refreshing the home page, or does rotation change periodically? Your answer will help me determine if it's better to do several short promotions or one longer one .

So this is a longer answer than you were probably looking for, but here goes:
Tim wanted a way for display time to be fair for the classified ads. If it was totally random then someone posting an ad for one day might get close to zero views by bad luck. On the other hand, if you pay for your ad to run longer, it should receive commensorately more views over the course of its run, since that's what you're paying for. He designed a system nicknamed "favor" to regulate the randomness. You can imagine favor like a bank account. We can pretend that when an ad first runs it has a hypothetical bank account. When the random ad picker looks for ads to display on the home page, it can be digitally "bribed" (note: NOT ACTUALLY BRIBED. THIS IS AN ANALOGY) to be somewhat more likely to pick any ad with favor remaining, and the favor decreases for each time this happens until it reaches zero - at that point the ad will still display, but with the exact same odds of displaying as every other ad with zero favor. What it amounts to is that one-day ads will get a good amount of views - probably more than true randomness would cause, but not exculsive display, since the favor only makes an ad more likely to be picked. The longer you request your ad to run, the more favor it is assigned. The net effect of all this is that ad display becomes self-regulating over time - everyone ends up with about the same homepage ad views per day. If an ad has been randomly picked a lot, it will run out of favor faster, and then it will start displaying less often. The reverse is true as well.